In the early stages of any race for the presidency, journalists and political scientists find themselves making predictions and extrapolating national trends from local results when very few of the American public have had a chance to vote.

It is crunching numbers, when there simply aren't enough numbers to crunch.

But this weekend, after the Republican primary in South Carolina and caucuses for both parties in the Western desert state of Nevada, we will have a little more hard information to work with.

For different reasons, the two states may help to give us a feeling for how the race is shaping up on the way into so-called Super Tuesday, when voters in 22 states will have their say, and the nominations just might be wrapped up.

I generally distrust psephological "rules" derived from previous elections... along the lines of "no Republican candidate from the South has placed better than 3rd in Wisconsin in a year when the Boston Red Sox won the World Series".

How dull the race would be if the past really was an infallible guide to the present.

Intriguing statistic

Having said that, it is impossible to ignore the importance to Republicans of South Carolina, the first of the states of the old South to vote.

Most voters have not yet had a chance to express their preference

Since 1980, the candidate it has picked has always gone on to win the party's nomination, even when, as with Bob Dole in 1996, he didn't go on to win the White House.

Any Republican in modern times who's won two out of three in Iowa, New Hampshire and South Carolina has always won the nomination.

With Mitt Romney, John McCain and Mike Huckabee all having secured early wins in different places, that statistic looks particularly intriguing.

This year, of course, the race is so open that conventional statistical yardsticks may cease to apply altogether.

The truth is, the Republican race remains a mess.

Dirty tricks

That is in part because one of the national front-runners, former New York Mayor Rudy Giuliani, has taken the extraordinary gamble of virtually ignoring the early races to concentrate on the bigger prize of Florida at the end of the month.

John McCain is hoping to regain momentum in South Carolina

But it's also because none of the Republican candidates has shown the ability to pull together the various threads that make up the party.

Each of them appeals to one section or another, whether it is Christian social conservatism (Huckabee), belief in big business (Romney) or experience in military and defence matters (McCain).

Victory will probably go to the candidate who manages to reach out to the rest of the party without alienating his own base.

And, of course, in politics when things are tight, they start to get a little dirty.

South Carolinians are no strangers to the dark arts of negative campaigning, and 2008 may turn out to be a year to remember on that front.

'Truth Squad'

Already I've been told stories about bogus Christmas cards circulating which purported to come from Mr Romney, a Mormon.

Mitt Romney may have suffered attacks based on his Mormon faith

They contained references to polygamy which purported to come from the Book of Mormon.

But wherever the quotes came from, the cards certainly didn't come from Mr Romney. And the Mormons discontinued polygamy well over a century ago.

And a shadowy group calling itself Vietnam Veterans Against John McCain has been busy too, handing out flyers implying that in Mr McCain's long spell as a prisoner of war in Hanoi, during which he was tortured, he gave information to America's enemy.

The evidence? Well, there isn't any. It's a straightforward attempt to take one of Mr McCain's best cards - his status as a war hero - and use it against him.

Something very similar happened to Mr McCain when he lost South Carolina to George W Bush in 2000, but this time he's mobilised a group of volunteers called "the Truth Squad" to lead his rebuttal of the rumours. We shall see how they fare.

Fast-growing Nevada

Nevada, by comparison, has been a model of decorum, but there is plenty at stake there too.

With the Republicans largely focusing on South Carolina - except for the eccentric Mr Giuliani, who's tirelessly flogging up and down the freeways of Florida - it is the Democratic race out West which is making headlines.

Essentially, the race boils down to another round in the evenly-matched struggle between Hillary Clinton and Barack Obama and, as the race is evenly-balanced nationwide, so it is proving in Nevada.

Each candidate can point to a poll which puts them ahead. Senator Clinton can point to backing from influential politicians in the Hispanic community, Senator Obama has the backing of a powerful local union organisation.

And the backdrop to the campaign is interesting too, in a state which for decades was the fastest growing in the whole of the US.

Nevada has by far the highest rate of foreclosure in the country - foreclosure being the brutal business whereby the bank takes your home off you if you cannot meet your mortgage payments.

American prosperity feels brittle and the electorate is nervous of recession - we can expect to hear in Nevada how the two leading Democrats would help the needy, and manage the economy.

On opposite sides of the country this weekend, there is a great deal at stake... and the prize of momentum to take forward first to Florida, then on to Super Tuesday itself.